1. Field of the Invention
This invention relates to a process for producing metal, ceramic and cermet articles from low viscosity suspensions and the articles produced thereby. The articles include micro diameter hollow fibers, tubes having hollow walls, solid and hollow sheets, and open cell foams. The articles are useful for filters, catalyst media, fuel cell electrodes, body implantation devices, structural materials, vibration and noise control, heat exchangers, heat sinks, heat pipes, heat shields and other applications.
2. Description of the Related Art
U.S. Pat. No. 4,268,278 described a method of preparing inorganic hollow fibers by first forming a polymeric precursor hollow fiber laden with the inorganic material, removing the polymer and sintering the inorganic material. The hollow fiber had a radially anisotropic void volume wall structure.
U.S. Pat. Nos. 5,011,566 and 5,298,298 described a method of preparing a microscopic tube by first depositing a oxidation resistant material on the surface of an oxidizable fiber, preferably by chemical vapor deposition, said deposition occurring in an inert environment. The coated fibers were placed in an oxidizing environment and the base fiber was removed by oxidation, the remaining coating forming a hollow tube.
U.S. Pat. No. 5,352,512 described a method of preparing a microscopic tube by first depositing a solvation resistant material on the surface of an dissolvable fiber, preferably by chemical vapor deposition, said deposition occurring in an inert environment. The coated fibers were placed in a solvating environment and the base fiber was removed, the remaining coating forming a hollow tube.
U.S. Pat. No. 6,194,066 described microscopic tubes having porous walls or multi-layer walls prepared by methods similar to those described in U.S. Pat. Nos. 5,011,566, 5,298,298 and 5,352,512.
U.S. Pat. No. 6,458,231 described methods of manufacturing microtubes that had peripheral geometries that were not uniform along the tube axis.
Methods for the preparation of porous ceramic bodies through coating of foam substrates with slurries of ceramic particles have been described for example in U.S. Pat. Nos. 3,090,094, 3,097,930, 3,893,917, 3,893,917, 3,947,363, 3,962,081, 3,993,495, 4,004,933, 4,024,212, 4,056,586, 4,075,303, 4,113,241, 4,154,689, 4,158,684, 4,265,659, 4,343,704, 4,610,832, 4,803,025, 4,833,106, 4,866,011, 4,885,263, 4,923,830, 4,975,191, 5,039,340, 5,177,035, 5,185,297, 5,429,780, 5,676,833, 5,705,118, 6,426,163, 6,932,925 and United States Patent Application 20040077480.
These prior art methods were limited in the combination of pore density and body thickness that could be achieved. U.S. Pat. Nos. 4,056,586 and 3,893,917 reported that the maximum body thickness that could be achieved at a pore density of 100 pores/in (39.37 pores/cm) was 10 cm. In U.S. Pat. No. 4,075,303 using a slurry viscosity of 1,000 to 80,000 centipoises (1 to 80 Pa-s), the maximum pore density that could be achieved at a thickness of 10 cm was 25-35 pores/in (9.84-13.78 pores/cm).
Methods for the preparation of porous metal or inorganic bodies through coating of foam substrates with slurries of metal particles or inorganic particles have similarly been described for example in U.S. Pat. Nos. 3,111,396, 3,408,180, 3,946,039, 4,560,621, 5,640,669, 5,881,353, 5,951,791, 6,399,528, 6,387,149, 6,524,522, 6,706,239, 6,840,978 B2, and Japanese Kokai Patent Publication JP6271904. JP6271904 described use of low viscosity slurries of metal particles in liquid phenolic resins.
The maximum body thickness that could be achieved with slurries of metal particles was reported in U.S. Pat. No. 5,640,669 to be 0.25 cm at a pore density of 50 pores/in (19.69 pores/cm).
By way of further background, the penetration of porous media by pure liquids is sensitive to the viscosity of the liquid as expressed by the Lucas-Washburn equation. To wit,
      x    2    =                    σ        ⁢                                  ⁢        r        ⁢                                  ⁢        cos        ⁢                                  ⁢        θ                    2        ⁢                                  ⁢        μ              ⁢    t  
where x is the distance penetrated by the liquid, σ is the surface tension of the liquid, μ is its viscosity, θ is the contact angle between the liquid and the surface of the capillary, r is the radius of the capillary and t is the penetration time. (Ref.: Marmur et al., “Characterization of Porous Media by the Kinetics of Liquid Penetration The Vertical Capillaries Model”, J. Colloid and Interface Sci., 199 299-304 (1997)). This sensitivity of penetration distance to viscosity appears not to have been recognized in the prior art cited above for preparing porous inorganic bodies by coating foam substrates.
Each of the prior art methods represented progress toward the goals to which they were directed. However, none described the specific methods or articles of this invention and none recognized the problem and satisfied all of the needs met by this invention. Prior art methods of producing hollow metal fibers produced fibers with radially anisotropic walls or they were limited in the length of the fibers that could be produced with impervious walls of uniform radial composition. A need exists for hollow metal fibers having isotropic impervious walls of uniform radial composition. Further, a need exists for continuous hollow metal fibers of indefinite length.
Prior art methods of producing porous metal or ceramic articles using slurries were limited in their ability to produce both high pore density and thick sections simultaneously. Some prior art methods used solvents that adversely affected the environment and/or left carbon residue in the final article. Prior art processes seem not to have produced continuous articles of indefinite length. A need exists for porous metal and inorganic articles that have high pore density in thick sections and are made using environmentally benign materials. A need exists for continuous porous metal and polycrystalline ceramic articles of indefinite length. Further, a need exists for metal and ceramic meshes, tubes and sheets having hollow walls. Other needs satisfied by the invention are improved methods of making metal foils, the foils thereby produced, and methods of making an inorganic coating on an inorganic substrate.